A crowd of thousands will line the quayside while hundreds of boats will form a "nautical red carpet". The celebrations in Sydney on Saturday promise to rival those of its spectacular new year displays as Australia welcomes home Jessica Watson, at 16 the youngest sailor ever to circumnavigate the globe single-handed.

Or is she? As the Queensland teenager's yacht, Ella's Pink Lady, sails into Sydney harbour after an epic seven-month voyage, the TV helicopters hovering above will not be the only thing clouding blue skies. Controversy hangs in the air.

Experienced old salts have queried whether Watson, who arrives back three days shy of her 17th birthday, has sailed far enough, claiming her route falls some 2000 nautical miles short of a true circumnavigation according to the rules set by the global authority, the World Sailing Speed Record Council. She didn't travel far enough north of the equator, they have calculated.

So she has failed, they declare, in her stated aim to beat the 1999 record of fellow Aussie Jesse Martin, which he set over 328 days at the age of 18.

It's a moot point. Following the furore over allowing ever younger sailors to undertake such a risky endeavour, sparked when the Dutch courts intervened last year to prevent Laura Dekker, 13, from setting sail solo, the WSSRC no longer recognises the category of "the youngest".

So Watson will claim no record. But at stake are the millions she could, potentially, earn as a result of her highly-publicised venture. One of Rupert Murdoch's Australian papers has bought her exclusive story for a reported A$700,000 (£430,000), and a TV station has exclusive live rights. Watson, who has been sailing since she was eight, will step off her yacht and into a whirlwind nationwide "Meet Jessica" tour. A book is due out in July.

Meanwhile her website is doing brisk trade in baseball hats, wall charts and other sailing paraphernalia. It boasts 14 major sponsors, and the apple growers Pink Lady Australian hope she will become the company's poster girl.

According to navigation experts at the respected Sail-world.com magazine, Watson will have travelled 18,265 nautical miles, but a valid circumnavigation requires 21,600.

That is not to detract from her otherwise remarkable achievement and raw guts, the magazine said.

But it added that, though her log may show she has travelled 23,000 nautical miles, as claimed by her PR team, this includes "tacking and gybing", and not the straight line distances required.

Their claims have led to ill-tempered exchanges with Watson's PR manager Andrew Fraser, and criticism from the many Watson supporters gripped by her journey.

"We are not really interested in the technical concerns of a minority," wrote Fraser in an email to Sail-world.com. "The rising tide of her supporters is a millions multiple of the minority." As the council wouldn't be recognising her record anyway, her team argue, she does not need to adhere to their rules. And by their calculation, she has met all the requirements. Watson has brushed off the controversy. "Call me immature but I've actually been having a bit of a giggle over the whole thing," she wrote on her blog. "If I haven't been sailing around the world, then it beats me what I've been doing out here all this time."

Critics say they do not blame Watson but rather her PR team for the ambiguity over the world record status. Sail-world.com points out that Watson's website had originally stated she had "set her sights on beating Jesse's record", but have now replaced the world "record" with "achievement".

Unofficial polls meantime show that 75% of Australians believe she is a record breaker – whatever the rules.

Watson sailed out under a cloud. She collided with a coal freighter during a test sail, leading to claims she was inexperienced. Critics say her feat could set a bad example and encourage even younger children, and her parents were decried for encouraging her.

But waiting for her on the quayside will be her British friend and fellow sailor Mike Perham, who last year became the youngest to sail around the world, aged 17. Jesse Martin will also be there – with his record still, seemingly, intact.

Extracts from Jessica Watson's blog

24 April 2010 Despite the fact that today started with a knockdown, a wet bunk, a headache and some pretty huge seas, I've had a great day.

8 April2010 Some bad news today … the handle has fallen off my only kettle.

22 Nov 2009 I've finally got a fish … I can't say there would be many fishermen who would be proud of the mess I made of filleting it.

17 Nov 2009 Let me introduce you to "Silly". He's a little brown seabird. Silly earned his nickname because of his fascination with the wind generator and his amusing attempts to land on the bendy windvane blade.

12 Nov 2009 I was told this was the most watched blog in Australia and stage fright has left me a bit speechless.

2 Nov 2009 After completing the first English assignment I sent it off to discover I'd opened the wrong bag and have been working on next year's assignments, oops!